1. Field of the Invention
The present invention relates to an improvement in rare gas-halogen excimer lasers, and in particular, to an improvement for increasing the operational lifetime, reliability, efficiency, and/or performance of such lasers.
2. Description of the Related Art
An excimer laser uses a rare gas such as krypton, xenon, argon or neon, and a halide gas or a gas containing a halide, for example F2 or HCl, as the !active components. The active components and possibly other gases are contained in a pressure vessel provided with longitudinally extending lasing electrodes for inducing a transverse electrical discharge in the gases. The discharge causes the formation of excited rare gas-halide molecules whose disassociation results in the emission of ultraviolet photons constituting the laser light. The laser further comprises mirrors or reflective surfaces that form an optical cavity that establish an optical resonance condition. The laser gases are circulated between the lasing electrodes by a fan and may be cooled by a heat exchanger, a structure that removes excess heat.
Excimer lasers emit pulses of ultraviolet radiation and have potentially many practical applications in medicine, industry and communications. This potential success has remained to a large extent unfulfilled because of numerous problems that limit the period of time during which excimer lasers will operate without requiring substantial maintenance or experiencing performance difficulties. One of the obstacles to achieving a practical excimer laser is the challenge of obtaining a homogeneous volumetric discharge between the longitudinally extending lasing electrodes. Preferably, the discharge between the electrodes is substantially evenly distributed within the space separating across the electrodes. The intensity of the discharge between the electrodes, however, can be significantly different at different locations. Such inhomogeneous arcing between the electrodes causes the eventual destruction of the electrodes as well as contamination of the laser gases and optics with sputtered electrode material.
In order to overcome this problem, pre-ionization of the gas volume has been provided. Pre-ionization creates a low level electron cloud prior to the laser-exciting electrical discharge. This pre-ionization results in a homogeneous discharge. One type of pre-ionizer uses a non-solid, perforated, metallic longitudinally extending electrode separated from a co-axial ground electrode by an insulator. The pre-ionizer electrodes are co-axially situated within one of the lasing electrodes, which is made of conductive screen or mesh. The voltage applied to the pre-ionizer electrodes creates a plasma around the pre-ionizer electrodes which produces ultraviolet radiation. The ultraviolet radiation passes through the screen of the surrounding longitudinal lasing electrode to the area between the lasing electrodes and ionizes a portion of the gas there, allowing for a homogeneous discharge when an electric pulse is applied to the lasing electrodes. These additional components within the laser cavity, however, may be potential sources of contamination of the laser gases. Contamination of the laser gases during the operation of an excimer laser may quench the laser action as described more fully below.
Another difficulty with conventional excimer lasers is that contamination of the laser gases or the optics in the pressure vessel necessitates that major maintenance and/or disassembly of the laser be frequently undertaken such as, for example, in the case where the windows need to be replaced. Currently, the operational lifetime of excimer lasers is on the order of about a few tens of millions of pulses per window change. At typical pulse rates of between about 10 and 500 pulses per second, the operating time between such maintenance procedures or disassembly is on the order of hours, rendering such excimer lasers impractical for many, if not most, applications. In addition, because the toxic and corrosive gases used in excimer lasers must be carefully handled during disassembly of the laser and subsequent reassembly, such procedures are complicated and potentially hazardous.
What is needed therefore are laser designs and methods that reduce contamination.